As a result of the increasing use of fossil fuels, the concentration of carbon dioxide in the atmosphere has risen from 280 ppm in pre-industrial times, to almost 400 ppm in 2013,1,2 leading to rises in average global temperatures. This is expected to increase further in the short to mid-term until energy supplies which do not result in significant CO2 emissions become established.3 According to the International Energy Agency World Energy Outlook (2002), the predicted increase in combustion generated CO2 emissions is around 1.8% per year and by 2030, if it continues at that rate, it will be 70% above 2000 levels.4 
Hence, without significant abatement of CO2 emissions, the global average temperature may increase by 1.4-5.8 K by 2100.5 In view of the abundant global reserves of coal, this fuel is widely used for power generation in many countries around the world. However, for each unit of electricity generation, combustion of coal produces approximately double the amount of CO2 when compared with natural gas. This problem is likely to be exacerbated in the future, because of the expected increase in coal burning for power generation units in order to sustain the economic growth of developing countries like China and India. Other substantial CO2 producers include cement manufacturers, and ammonia production plants. Nevertheless, the major problem arises from coal fired power stations, with over 33% of current global CO2 emissions arising from such plants, and this high percentage offers a real opportunity for the reduction of CO2 emissions by capturing CO2 at source,6 concentrating it, and then handling it by storage in geological features (e.g. natural gas wells or the seabed), enhanced oil recovery, or sequestration—most likely by chemical or biochemical conversion into useful products (e.g. formic acid, methanol, polycarbonate plastics, polyhydroxyalkanoates, and biofuels).
The main current approach to absorption and stripping of CO2 in packed columns is considered to be a mature technology, typically using aqueous monoethanolamine (30% w/w), or related amine blends, as absorption media.4,5,7 However this approach has considerable problems, particularly when used to treat large volumes of flue gas with low CO2 concentrations (typically 3-5% for natural gas and 10-15% for coal combustion), as the processes require the use of large sized equipment, thereby representing a major engineering challenge with high investment costs. The current processes are also highly energy intensive, usually requiring the temperature of the whole capture solvent to be raised from about 40° C. to around 120° C., the latter temperature being attained under pressure. The operation of processes at such temperatures and pressures also promotes solvent degradation, corrosion of equipment and environmental emissions.
Typical apparatus used in a conventional Post-Combustion Capture (PCC) process is shown in FIG. 1, wherein it will be seen that a lean solvent is contacted with a cooled flue gas in an absorber. The CO2 is absorbed from the flue gas into the solvent, the resulting rich solvent is pumped through a lean-rich heat exchanger where the rich solvent is heated towards stripping temperatures of typically 90° to 115° C. by absorption of heat from the lean solvent. The CO2 is then desorbed in the stripper with the aid of steam, the lean solvent is cooled with the rich solvent in the lean-rich heat exchanger, and the solvent is then further cooled to absorption temperature and fed back to the absorber. The CO2 released from the stripper is subsequently cooled and the water vapour is condensed. Then the CO2 is compressed, typically in six stages, from stripper pressure to the pressure required for, e.g. a CO2 pipeline.
In a coal-fired power plant, typical energy consumption in the stripper reboiler can be as high as 15-30% of the power production. As a consequence, it has been calculated that application of current CO2 capture technology to power plants would increase the price of electricity by as much as 70%.2 In addition, the scale of CO2 capture technology has to be potentially enormous to deal with the large volumes of flue gases to be processed. A large power station such as Drax in Yorkshire UK produces approximately 55,000 tonnes of CO2 per day.8 This corresponds to a volume of around 28M m3 at atmospheric pressure which would require processing on a daily basis. On the basis that CO2 represents 10-15% of a typical flue exhaust form coal firing the actual volume of gas to be processed would be typically 7-10 times this amount.2 
In principle, the gas separation technologies which are currently used in the chemical industry, such as absorption in chemical solvents, adsorption using a solid adsorbent, membrane separation and cryogenic processes, can all be adapted for post-combustion capture of CO2 from thermal power plants. Alternative approaches such as pre-combustion CO2 capture, as in an integrated gasification and combined-cycle (IGCC) plant, and combustion using pure oxygen instead of air (known as oxyfuel combustion) for the production of sequestration-ready CO2, are also being developed for this purpose.7 
However, such technologies are either not yet fully developed for deployment (a number of demonstrator plants are currently in production), are not suitable for CO2 removal from flue gases emanating from large power plants, or cannot be retrofitted to existing facilities. Consequently, the preferred option in the immediate future seems to be the post-combustion capture of CO2 via absorption (scrubbing) in amine-based solvents with solvent regeneration by steam stripping, because this is already a well-established process which finds widespread use in the chemical industry.4,5 
Although absorption/stripping is a mature technology,9 it suffers from considerable problems when used to treat large volumes of flue gas. Despite widespread use of this technology, the underlying chemistry is only recently becoming more fully understood, mainly because of the complex behaviour of aqueous amine based systems.10 The situation is further complicated by recent developments utilising mixed aqueous amine systems such as monoethanolamine (MEA) and N-methyldiethanolamine (MDEA),11 and other blends containing, for example, piperazine (PZ) and 2-amino-2-methyl propanol (AMP).12 However, whilst these materials give more favourable energy considerations, their relative expense, stability and volatility present potential drawbacks, and the energy requirements are still too high.12,13 
Currently, aqueous MEA is widely used for CO2 capture, and it typically serves as a benchmark for comparison with potential new systems; it also highlights some important issues with amine based approaches. Thus, it is known that MEA degrades after prolonged use, particularly due to the presence of residual oxygen in the flue gas stream. It is also important that the cost of solvent make-up should not be excessive in a viable commercial process. A wide variety of other solvents is also available, and the relative merits of these solvents, and other aspects, have been recently assessed.12 Ammonia14 would appear to offer some advantages over MEA and other amines in aqueous based systems, in terms of energy requirements, stability and disposal, although emissions control may be more challenging.
A consideration of the chemistry of amine-based solvents shows that there are three main routes by which amines can absorb CO2, as illustrated in Scheme 1.2,10 

The particular mechanism which operates in any given situation depends on process considerations such as the presence of water or solvent, the concentration of amine and its structure, pH, and CO2 concentration and pressure. In aqueous based systems it is likely that all three mechanisms are operating, but that the overall mechanism involves predominantly the carbamate salt and ammonium bicarbonate.10 The carbamic acid is often favoured in solvents of high polarity (e.g. DMSO) but, otherwise, the ammonium carbamate is the dominant species in non-aqueous environments. All the CO2-amine adducts decarboxylate on heating, liberating CO2 and regenerating the amine. For example, in the case of aqueous MEA, decarboxylation is typically carried out at 120° C. at 0.2 MPa, which has significant energy implications for the overall process.
In addition to amines, a range of other organic molecules can reversibly capture CO2, often when they are converted into basic salts, or if they have inherent basicity themselves. The most appropriate method for determining the suitability of a molecule for CO2 capture is based on its pKa—that is, the acidity of the conjugate acid.
pKa is defined as the −log of Ka, the acid dissociation constant, and is derived from the following equations:
            For      ⁢              :            ⁢                          ⁢      HA        ⇌                  H        +            +              A        -                        K      a        =                            [                      H            +                    ]                ⁡                  [                      A            -                    ]                            [        AH        ]                        pK      a        =                  -        log            ⁢                          ⁢              K        a            where HA represents the acid species and the quantities in square brackets are concentrations. Values quoted are usually measured in water (hence H+ is usually present as the hydronium ion, H3O+), unless otherwise stated, but can be very solvent dependent (vide infra).
Other than simple primary amines, most solvents for CO2 capture operate via the bicarbonate salt route, where CO2 is hydrated by water, facilitated by the basic solvent. The effective pKa of CO2 in water is 6.3, which arises from a combination of the various equilibria when CO2 is dissolved in water:H2O+CO2H2CO3 Kh=1.70×10−3 H2O+H2CO3H3O++HCO3−pKa1=3.60H2O+HCO3−H3O++CO32−pKa2=10.332H2O+CO2H3O++HCO3−pKa=6.30Thus, suitable bases need to have a pKa significantly higher than this in water, and usually the value should be in the range of 8-12. In aqueous systems pH also plays a major role in determining active species in solution and the effectiveness of the capture and release process. Again, an alkaline pH of 10-13 is usually required for efficient absorption, and is usually controlled by the addition of base (e.g. hydroxide or amines) to the solution. As these solutions absorb CO2, the pH decreases as neutralisation of the base takes place. Heating the CO2 loaded solution liberates CO2 and regenerates the basic solution for re-use. Alternatively, significantly lowering the pH of the solution (e.g. by the addition of acid) also induces rapid decarboxylation via protonation of bicarbonate to form carbonic acid, which undergoes rapid conversion back to CO2 and water.
Acid gas capture is a very active field of research and extensive work is underway using bases, typically amines, which react with CO2, and which is then liberated on heating to regenerate the original solvent. Whilst there are many different variations on this theme, almost all of the prior art uses the thermal approach for decarboxylation and solvent regeneration.
Recent studies using alcohols (or thiols) and appropriate bases shows considerable promise, but require anhydrous conditions, which is a major limitation for typical flue gas streams.15 Related disclosures include WO-A-2008/068411, which teaches the use of an amidine or guanidine base and a hydroxyl or thiol, and U.S. Pat. No. 7,799,299, which discloses acid gas binding organic liquid systems (CO2-BOLS) that permit separation of one or more acid gases, again based on amidines and guanidines reacting with weak acids, such as alcohols and thiols, in the presence of acid gases. WO-A-2012/031281 teaches related chemistry where a nitrogenous base reacts to form a carbamate salt or heteroatom analogue, without any substantial formation of a carbonate ester or heteroatom analogue.
Amongst other approaches to the capture of CO2, US-A-2006/0154807 discusses a boronic acid-derived structure comprising a covalently linked organic network including a plurality of boron-containing clusters linked together by a plurality of linking groups which may be used to adsorb carbon dioxide. Similarly, WO-A-2008/091976 relates to the use of materials that comprise crystalline organic frameworks, including boronic acid derived-structures, which are useful for the storage of gas molecules, such as CO2. GB-A-1330604, on the other hand, is concerned with the separation of carbon dioxide from a gas stream by scrubbing with an aqueous solution of orthoboric acid and potassium hydroxide at 70° to 160° C. at a pressure from 1 to 30 atmospheres.
US-A-2005/0129598 teaches a process for separating CO2 from a gaseous stream by means of an ionic liquid comprising an anion having a carboxylate function, which is used to selectively complex the CO2. The ionic liquid, which is effectively a low melting molten salt made up entirely of ions, can subsequently be readily regenerated and recycled.
GB-A-391786 discloses a process for the separation of carbon dioxide by means of aqueous solutions containing alkalis in chemical combination with sulphonic or carboxylic organic acids, including amino-sulphonic acids, amino acids such as alanine and asparagines, mixtures of amino acids obtained by the degradation of albumens, weak aliphatic mono- and di-carboxylic acids, and imino acids such as imino di-propionic acid. The hydroxides and oxides of sodium, potassium, lithium, or salts of these metals such as the carbonates, are preferably used as the bases.
U.S. Pat. No. 1,934,472 teaches a method for the removal of carbon dioxide from flue gases which involves treating the gas mixture with a solution of sodium carbonate or triethanolamine carbonate, and subsequently liberating the carbon dioxide by heating the resulting liquid under reduced pressure.
U.S. Pat. No. 1,964,808 recites a method for the removal of carbon dioxide from gaseous mixtures which involves treating the mixtures with a solution of an amine borate and subsequently liberating the carbon dioxide by heating the resulting liquid.
U.S. Pat. No. 1,990,217 discloses a method for the removal of hydrogen sulphide from gaseous mixtures which involves treating the mixtures with solutions of strong inorganic bases, such as alkali metal or alkaline earth compounds, with organic acids containing carboxylic or sulphonic acid groups and, if desired, liberating the hydrogen sulphide by heating.
U.S. Pat. No. 2,031,632 is concerned with the removal of acidic gases from gaseous mixtures by treating the mixtures with solutions of basic organic amino compounds, such as ethanolamines, in the presence arsenic or vanadium compounds, and the liberation of the acidic gases by heating.
GB-A-786669 relates to the separation of carbon dioxide or hydrogen sulphide from a gaseous mixture by a process using an alkaline solution containing an amino acid or protein under pressure and at elevated temperature, whilst GB-A-798856 discloses the separation of carbon dioxide from a gaseous mixture by means of an alkaline solution containing an organic or inorganic compound of arsenic, in particular arsenious oxide as such, or as arsenite. In each case, regeneration may be effected by passing hot air or steam through the solution, and the alkaline solution may contain sodium, potassium or ammonium carbonate, phosphate, borate, arsenite or phenate or an ethanolamine, whilst boric acid, silicic acid, and salts of zinc, selenium, tellurium and aluminium act as synergistic agents for the arsenious oxide.
Similarly, GB-A-1501195 relies on a process using an aqueous solution of an alkali metal carbonate and an amino acid, for the removal of CO2 and/or H2S from gaseous mixtures, the improvement on this occasion involving the addition of compounds of arsenic and/or vanadium to the absorbing solution as corrosion inhibitors. Again, regeneration of the gases is subsequently effected.
U.S. Pat. No. 2,840,450 teaches the removal of carbon dioxide from gaseous mixtures by a method which involves treating the mixtures with an alkaline solution of an aliphatic amino alcohol, carbonate, phosphate, borate, monovalent phenolate or polyvalent phenolate of sodium, potassium or ammonia in the presence of selenious acid or tellurous acid or their alkali metal salts, and subsequently liberating the carbon dioxide by heating the resulting liquid.
U.S. Pat. No. 3,037,844 recites a method for the removal of carbon dioxide from gaseous mixtures which involves treating the mixtures with an aqueous solution of a carbonate, phosphate, borate, or phenolate of an alkali metal or ammonia in the presence of arsenious anhydride, and subsequently liberating the carbon dioxide.
GB-A-1091261 is concerned with a process for the separation of CO2 and/or H2S from gaseous mixtures which requires passing the mixture through an absorbent liquor comprising an aqueous solution of an alkali metal salt of a weak acid, such as potassium carbonate or tripotassium phosphate, and then passing the liquor containing dissolved acidic gases into a regenerator where the liquor is heated and stripped with steam to liberate the acidic gases.
U.S. Pat. No. 4,217,238 relates to the removal of acidic components from gaseous mixtures by contacting aqueous solutions comprising a basic salt and an activator for the basic salt comprising at least one sterically hindered amine and an amino acid which is a cosolvent for the sterically hindered amine.
U.S. Pat. No. 4,440,731 teaches corrosion inhibiting compositions for use in aqueous absorbent gas-liquid contacting processes for recovering carbon dioxide from flue gases, the method employing copper carbonate in combination with one or more of dihydroxyethylglycine, alkali metal permanganate, alkali metal thiocyanate, nickel or bismuth oxides with or without an alkali metal carbonate.
Likewise, U.S. Pat. No. 4,446,119 is concerned with a corrosion inhibiting composition for the separation of acid gases such as carbon dioxide from hydrocarbon feed streams which, on this occasion, contains a solution of e.g. an alkanolamine with water or organic solvents and small amounts of soluble thiocyanate compounds or soluble trivalent bismuth compounds, with or without soluble divalent nickel or cobalt compounds.
CN-A-102764566 describes the use of a 1,2,4-triazole salt with imidazolium or tetrazole salts for capture of acid gases, including CO2, in water or a range of other solvents, whilst WO-A-2009/066754 reports the use of imidazoles in combination with amines for the capture of CO2.
U.S. Pat. No. 4,624,838 discloses a process for removing acid gases using either a 5 or 6-membered heterocyclic ring having a pKa of no greater than about 8, with imidazole as the preferred heterocyclic nitrogen compound, and U.S. Pat. No. 4,775,519 discloses a mixture of N-methyldiethanolamine with imidazole or a methyl substituted imidazole to capture CO2.
Triazole salts alone have also been reported for use in non-aqueous systems (particularly DMSO), which overcomes some of the solubility issues.17 In this case, it is reported that CO2 capture occurs by direct reaction of triazolate with CO2 forming a new covalent N—C bond; this is a different mechanism to that likely to operate under aqueous conditions, where capture most probably occurs via bicarbonate formation. Ionic liquid triazole salts have also been disclosed as CO2 capture agents.18 
Guanazole has been used as a ligand for catalysts for electrochemical reduction of CO2,19 and in metal-organic frameworks (MOFs) which have potential as solid absorbents for CO2 capture, but these materials have very different mechanisms by which they operate and methods of commercial implementation.20 
In WO-A-2011/135378 there is disclosed a method for the capture of carbon dioxide gas which comprises contacting the carbon dioxide with a composition comprising at least two compounds selected from basic compounds, at least one of which is an organic compound and at least one of which is an inorganic salt. Typically, the basic organic compounds may comprise amino compounds, or salts obtained by treatment of weakly acidic organic compounds with bases.
Further alternative methods for CO2 separation have been reviewed, and a comparison of these suggests that membrane diffusion is potentially the most powerful method but requires suitable membrane materials to be developed.16 
It is apparent, therefore, that thermal liberation of CO2 and solvent regeneration is the main approach used in the prior art acid-base capture processes. In some cases, the temperatures required can be substantially less than for conventional amines, but the processes still require major energy input or have otherwise limiting aspects to their technology. Alternative methods for liberating CO2 and regenerating solvent are very limited.
A notable exception, however, is disclosed in WO-A-2006/082436, which relates to a gas separation device for separating a reactive gas—such as CO2—from a gaseous mixture, the device comprising porous anode and cathode electrodes separated by an ionic membrane, the anode being impregnated with an absorbent compound or solvent, whilst the cathode is impregnated with an electrically conductive liquid. Amongst suitable absorbent compounds for this purpose are amines, sulphonic acids and carboxylic acids. Absorption, desorption or both are promoted by application of electric charge to the electrodes. A recent study disclosed by M. C. Stern, F. Simeon, H. Herzog and T. A. Hatton, Energy Environ. Sci., 2013, 6, 2505, also describes electrochemically mediated amine regeneration by reduction of metal cations which otherwise react with amines and displace CO2.
Nevertheless, it is clear that current methods for capture of CO2 and other acid gases are expensive and far from ideal for large scale application, so the present invention attempts to address this problem by providing a solution which is relatively simple, and uses inexpensive processes and consumables. The process of the present invention seeks to provide lower energy requirements for decarboxylation and can operate at much lower temperatures (preferably ambient temperature), which minimises atmospheric emissions and degradation. These aspects are an increasing concern with amine-based systems, as is becoming more apparent in the light of recent pilot studies, particularly with regard to large scale implementation in areas such as power stations, cement and steel manufacture, brewing, and large scale chemical processes such as ammonia production.
The capture systems envisaged by the present invention would also be suitable for use in smaller scale specialist applications, such as in submarines, spacecraft and other enclosed environments. Furthermore, in view of the potentially dramatic increase in efficiency of the process of the invention when compared with conventional processes, the technology also has potential application to the capture of CO2 directly from the atmosphere. The use of the technology for the capture of other acid gases (e.g. H2S and SO2) or mixtures thereof is also envisaged, in applications such as natural gas sweetening and desulphurisation.
Whilst it is known that manipulation of the acidity (pKa) of acids and bases can be achieved by variation of solvent composition, it is surprising that this can be adapted in order to provide a highly efficient process for CO2 capture and release, wherein variation of solvent composition provides the required transformation in conditions between capture and release operation (for example, by switching the relative pKa of bicarbonate and a capture agent), rather than requiring the thermally driven processes of the prior art. Such a process can also be used in conjunction with other mechanisms, such as thermal release, but under much milder and more controlled conditions than previously required. In an alternative embodiment, the solvent composition can be controlled so as to optimise the acid base equilibrium in a capture and release process, such that non-conventional capture agents can be used and the solvent composition can be optimised to minimise the energy input requirements in the capture/release cycle.